A typical stent is a cylindrically shaped device, which holds open and sometimes expands a segment of a blood vessel or other anatomical lumen such as urinary tracts and bile ducts. Stents are often used in the treatment of atherosclerotic stenosis in blood vessels. “Stenosis” refers to a narrowing or constriction of the diameter of a bodily passage or orifice. In such treatments, stents reinforce body vessels and prevent restenosis following angioplasty. “Restenosis” refers to the reoccurrence of stenosis in a blood vessel or heart valve after it has been subjected to angioplasty or valvuloplasty.
A stent is typically composed of scaffolding that includes a pattern or network of interconnecting structural elements often referred to in the art as struts or bar arms. The scaffolding can be formed from wires, tubes, or sheets of material rolled into a cylindrical shape.
In the case of a balloon expandable stent, the stent is mounted on a balloon connected to a catheter. Mounting the stent on the balloon typically is a two-step process. First, the stent is compressed or crimped onto the balloon. Second, the compressed or crimped stent is retained or secured on the balloon. The retained stent should have a sufficiently small diameter so that it can be transported through the narrow passages of blood vessels. The stent must be secured on the balloon during delivery until it is deployed at an implant or treatment site within a vessel in the body of a patient. The stent is then expanded by inflating the balloon. “Delivery” refers to introducing and transporting the crimped stent through a bodily lumen to the treatment site in a vessel. “Deployment” corresponds to the expanding of the crimped stent within the lumen at the treatment site. Delivery and deployment of a stent are accomplished by positioning the stent about one end of a catheter, inserting the end of the catheter through the skin into a bodily lumen, advancing the catheter in the bodily lumen to a desired treatment location, inflating the stent at the treatment location, and removing the catheter from the lumen by deflating the balloon.
The stent should be firmly secured to the balloon to avoid detachment of the stent before it is delivered and deployed in the lumen of the patient. Detachment of a stent from the balloon during delivery and deployment can result in medical complications. A lost stent can act as an embolus that can create a thrombosis and require surgical intervention. For this reason, a stent must be securely attached to the catheter. Stent retention is greatly facilitated by protrusion of the balloon into the interstitial spaces or gaps between stent struts in a stent pattern. Preferably, a portion of a balloon extends from the interior of the stent through a stent gap to the exterior of the stent. In other words, the portion of the balloon preferably extends beyond the outer surface of the stent.
One method of retaining or securing a stent on a balloon is to use a split mold. The split mold includes two half-molds that together form a cylindrical chamber for accommodating the stent and balloon. The split mold can be opened by separating the half-molds from each other so that the stent and balloon can be placed in the chamber. Then the half-molds can be pressed together to secure the stent and balloon in the chamber. Next, the balloon is inflated with a pressurized gas to press portions of the balloon into the spaces or gaps between stent struts to retain the stent on the balloon.
A disadvantage of the conventional split mold is that the balloon cannot extend through the gaps of the stent and beyond the outer surface of the stent. The cylindrical chamber's surface, which supports the stent during balloon inflation, presses against the outer surface of the stent when the balloon is inflated, preventing the balloon from extending beyond the outer surface of the stent to enhance stent retention.